Most streets, parking, lots, and just about any other paved surface comprise storm water drains so that liquids, mostly water from rain or melting snow, will drain away instead of accumulating on the surface. Often these surfaces have a slight grade to guide water to the drains.
Of course, water is not the only thing flushed towards the drain. The flowing water washes all types of refuse towards the drain. Leaves, plastic bags, disposable cups, soda cans, and twigs, are all common litter that could flow into a drain, collect in the drain or further down the sewer, and cause a blockage. Often this risk is mitigated by a storm drain grate which covers the drain and blocks the largest refuse from entering the drain.
A simple storm drain grate is, however, largely ineffective at removing smaller materials and wholly ineffective at removing chemical, biological, or petroleum contaminants. Storm water inlet filters are known in the art, but the added maintenance requirements and often prohibitive costs have impeded acceptance.